The present invention relates to communications. More particularly, the present invention relates to a system and method for active impedance matching in communications systems.
In communications systems, it is important that the impedance of a load (e.g., an antenna or a cable) connected to a communication device be matched to the impedance of the device. A mismatch of load and device impedances causes power reflections at the point of connection that result, for example, in reduced efficiency, bandwidth and in reduced signal-to-noise ratio. Thus, if the impedance of a load does not match the impedance of a communications device, the load is typically connected to the device using an impedance matching circuit.
A particular problem encountered in wireless communications systems is that the impedance of an antenna typically changes with time due to the changing presence of obstacles (e.g., humans) in the vicinity of the antenna. In the frequency range from about 20 MHz up to about 1.5 GHz, the human body acts predominantly as a reflector with varying degrees of efficiency. Thus, the presence of one or more human bodies nearby an antenna will produce an increase in the capacitive nature of the antenna impedance, and thereby generate an impedance mismatch at the point of connection between the antenna and a wireless communications device.
As would be known to a person skilled in the relevant communications art, a human body proximate to an antenna affects the impedance of the antenna. As a person moves either closer to or further away from an antenna, the change in the relative position of the person proximate to the antenna causes the impedance of the antenna to change. A human body close to an antenna presents a low impedance to an RF wave. The presence of a human body reduces the electric field of an RF wave close by, and it increases the magnetic field. At a distance of a quarter wavelength away from the body, a high impedance is presented to an approaching RF wave front, which enhances the electric field while reducing the magnetic field. This effect is periodic, and it repeats each quarter of a wavelength of the RF wave. Furthermore, on the far side of a human body from a transmitter, there is a deep null in a transmitted RF wave caused by the absorption of the wave by the human body. Absorption affects both the electric field and the magnetic field of an RF wave.
While impedance matching circuits are known in the communications technology, the impedances of these known circuits do not change with time in order to match a load whose impedance changes with time. Consequently, in order to minimize the effects of load (antenna) impedance mismatch, wireless communication systems are typically over designed. Generally speaking, because of the nature of wireless communications links, it is impossible to predict where and under what environmental conditions a wireless communications device and its antenna will operate. As a result, in a well designed wireless communications system, an antenna should be insensitive to the presence of nearby objects and, in particular, to the presence of a human body. This requirement sets constrains on the antenna quality factor (Q) (i.e., Q must be designed to have less than a given value). In a wireless communications system, the Q of an antenna should be designed so that it satisfies EQ. 1:
Q less than (C/xcex94C)xe2x80x83xe2x80x83EQ. 1 
where C is the tuning capacitance, and xcex94C is the change of capacitance induced into the antenna, for example, by the proximity of a human body. For an air cored loop antenna, a typical value for C is 16 pF, and a typical value for xcex94C is 0.2 pF (i.e., Q must be less than 80). The typical resistance of a small loop antenna is very low (e.g., less than 2 xcexa9), and thus an impedance matching circuit is required even for free space radiation.
As would be understood by a person skilled in the relevant communications technology, the over design of wireless communications systems increases the cost of wireless communications. It also reduces the operational life time of portable wireless communication devices due to higher energy consumption requirements and battery drain.
What is needed is a means for matching the impedance of a load to the impedance of a communications device, which overcomes the deficiencies of known impedance matching circuits.
The present invention is directed to a system and method for active impedance matching in communications systems. A signal sensing module senses an RF signal and produces one or more secondary signals representative of the RF signal. An impedance matching control module generates a control signal, based on the one or more secondary signals, which is indicative of an impedance mismatch between a load and a communications device. The control signal is then applied to at least one variable impedance device to adjust the impedance of an impedance matching network and thereby reduce the impedance mismatch between the load and the communications device.
In an embodiment, a barium strontium titanate, thin film, parallel plate capacitor is used as a variable impedance device to adjust the impedance of the impedance matching network. In other embodiments, other variable impedance devices such as other types of thin film capacitors or varactor diodes can be used to adjust the impedance of the impedance matching network.
A feature of the present invention enhances the bandwidth and/or signal-to-noise ratio of a communications system.
Another feature of the invention enhances the operating range and battery life of portable wireless communications devices.
Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with reference to the accompanying drawings.